A passive optical network (PON) is shared between multiple optical network units (ONUs), where each ONU must transmit data to the common channel only during assigned time slots. If these rules of operation are violated, the integrity of the system transmission can be severely compromised.
An exemplary diagram of a typical PON 100 is schematically shown in FIG. 1. The PON 100 includes a number M of ONUs 120-1, 120-2, through 120-M, coupled to an optical line terminal (OLT) 130 via a passive optical splitter 140. Since all ONUs function in like manner, they will be collectively referred to by the reference numeral 120 in the following description unless reference is made to a specific ONU. Traffic data transmission may be achieved using ATM cells over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, downstream transmission from the OLT 130 is broadcast to all the ONUs 120. Each ONU 120 filters its respective data according to, for example, pre-assigned ATM VPI/VCI values. The OLT 130 transmits downstream data to the ONUs 120 and receives upstream data sent to the OLT 130 from ONUs 120. The OLT 130 broadcasts data to the ONUs 120 along with the shared data so that all the ONUs 120 receive the same data. On the other hand, each of the ONUs 120 transmits respective upstream data to the OLT 130 during different time slots allocated by the OLT 130.
The OLT 130 and each of the ONUs 120 include an optical transceiver for transmitting and receiving optical signals that encapsulate the data. FIG. 2 schematically shows a transmitter 200 of an optical transceiver that includes a laser diode driver 210 that drives a laser diode 220, the output signal of which is monitored by a photodiode 230. The photodiode 230 produces a current (IMONITOR) in proportion to the amount of light emitted by laser diode 220. The transceiver in each ONU 120 operates in a burst mode, i.e., the laser diode 220 outputs optical signals only during assigned time slots.
It will be understood that during transmission of digital data, the laser diode transmits two logic levels corresponding to logic “high” and logic “low”. When the laser diode is ON, the power level of the laser signal varies according to whether logic “high” or logic “low” signals are transmitted. Even when the laser diode is OFF, some residual power is transmitted by the laser diode and so its power level is not exactly zero. Two values are used to control the power level of the output optical signal: average power (AP) and extinction ratio (ER). The AP is the average power of light emitted by the laser diode 220 when high logic level and low logic level signals are transmitted during ON time. The ER is the ratio between light illuminated at “high” and “low” times. That is,
                    A        ⁢                                  ⁢        P            =                        (                                    P              Low              ON                        +                          P              High              ON                                )                2              ;    and              E      ⁢                          ⁢      R        =                  P        Low        ON                    P        High        ON            
The PONLow and PONHigh are the respective low logic level and high logic level optical power levels during ON time. One type of failure that causes an ONU 120 to always transmit data or noise to the common channel is know as “rogue ONU”. This may result from a laser diode 220 that has some radiant power during OFF time. In the related art, techniques for detecting rogue ONU failures are based on measuring the AP and comparing the AP value to a predefined threshold. If that AP exceeds the threshold value, the laser diode 220 is shut down. An example for such technique is disclosed in U.S. Pat. No. 6,650,839 which is incorporated herein by reference for its useful background descriptions of the state of the art heretofore. The disadvantage of the detection of rogue ONU based on the AP is the substantial delay in generating an indication on such failures. This delay results from the time required for measuring the AP value, which is typically done using a low pass filter after the energy in the photodiode 230 is discharged. Another disadvantage of the AP based detection is the inability to determine the power levels of high and low levels at ON times, and to adjust these levels to achieve proper operation of the transmitter 200.
Another type of failures that may be detected by using the AP relate to eye safety hazards. Safety standards demand that optical devices automatically detect such hazards in order to prevent eye injuries resulting from a laser diode transmitting high optical power. This may occur due to an electric short in the laser diode 220 or a disconnected photodiode 230.
It would be advantageous to provide a solution that enables reliable and fast detection of optical failures of PONs.